Wideband, high-data-rate wireless communications networks based on cellular technologies are used worldwide for delivering an ever increasing amount of information to a mobile society. According to fundamental principles of cellular technology, a coverage area is divided into multiple cells that are mutually arranged to communicate with mobile stations or devices with minimal interference. Communications from a mobile station crossing the coverage area is handed-off between adjacent cells according to the location of the mobile station within the coverage area. Each of the cells is generally served by a base station having a transceiver that communicates with the mobile device. The frequency spectrums of the communications signals associated with the cells are divided into multiple different frequency bands. Therefore, filters, such as passive microwave filters, are used to perform band pass and band reject functions for separating the different frequency bands.
Cell sizes are often reduced as information bandwidth handled by the cells increases. As a consequence, additional cells are required within a coverage area to provide wireless communication service to an increasing number of mobile stations. Increasing numbers of passive microwave filters are included in tower-mounted amplifiers and related equipment to address the bandwidth increases.
Conventional microwave filters include a metallic shell or filter body having dividing walls that partition an open interior space into recesses and a cover that closes the recesses to define air-filled filter cavities or resonators. The metalworking process forming the filter body must accommodate precise dimensioning of the recesses to achieve satisfactory filter performance. Typically, the filter body is formed by casting and the cover is formed separately by either casting or stamping. After forming, the filter body may require additional machining for tuning the resonators as desired.
The cover and filter body are assembled together to complete the microwave filter. A seam is defined about the contacting circumferences of the filter body and the cover. After assembly, the cover must have a good electrical contact with the filter body along the entire extent of the seam to ensure proper filter operation. If the microwave filter is exposed to an outdoor environment, the seam must be hermetically sealed against the infiltration of water and other elements so that the resonators remain moisture-free. The presence of moisture in the resonators reduces the long-term reliability of the microwave filter.
Generally, such conventional microwave filters are relatively expensive to manufacture. In particular, the need to manufacture the precisely dimensioned resonators and a separate cover increases the cost as each component must be individually manufactured and assembled together.
The physical size of conventional microwave filters may be reduced by loading inserts of a temperature stable ceramic material characterized by a high dielectric constant and a high quality factor into the recesses previously filled with air. However, despite the reduction in size, the manufacturing cost is not significantly reduced as the microwave filter still includes a filter body and cover, and the ceramic inserts must be loaded into the recesses within the filter body.
Additionally, to address the cost issue, certain microwave filters incorporate commercially-available metallized ceramic resonators into a low-precision, low-cost sheet metal filter body. The presence of the ceramic reduces the size of the microwave filter. However, such composite structures lack the relatively-low insertion losses and relatively-high rejection numbers required for tower-mounted amplifiers currently used in wireless communication networks. Therefore, filter performance suffers.
Therefore, it would be desirable to provide a microwave filter which addresses the problematic seams and cost issues associated with precision formed filters. It would also be desirable to address the performance disadvantages associated with low-cost conventional microwave filters.